1. Field of the Invention
The present invention relates to a hydrogen storage alloy which can electrochemically absorb and desorb hydrogen, an electrode comprising the alloy as an active material and a cell comprising the electrode.
2. Description of the Related Art
Since a nickel-cadmium cell which is widely used as one of alkaline cells uses cadmium as a negative electrode active material, environmental pollution caused by cadmium is being accused. Then, a new type cell which causes less pollution and can be used as a substitute cell for the nickel-cadmium cell is desired.
In order to make electronic devices small and cordless, a small secondary cell having a large capacity is required.
In view of these requirements, a nickel-metal hydride cell comprising a hydrogen storage alloy as a negative electrode material has been extensively studied.
As the hydrogen storage alloy, Ti-Ni alloy systems, Zr-Mn alloy systems and rare earth-Ni alloy systems are well known.
While the Ti-Ni alloy system and Zr-Mn alloy system have a comparatively larger capacity, they do not have sufficient properties at a high rate discharge, or their discharge at low temperatures around -20.degree. C. is difficult.
Since the rare earth-Ni alloy system has a smaller capacity than the Ti-Ni alloy system or Zr-Mn alloy system, the nickel-metal hydride cell using the rare earth-Ni alloy system has only the same weight energy density as the high capacity nickel-cadmium cell, and it is required to increase its capacity and energy density.
To increase the capacity and energy density of an alkaline cell, it is necessary to increase the energy density of at least one of positive and negative electrodes. Insofar as nickel hydroxide is used in the positive electrode of the nickel-metal hydride cell, it is difficult to increase the energy density of the positive electrode greatly. Therefore, it is essential to increase the capacity and energy density of the negative electrode.
To obtain a hydrogen storage alloy, usually at least one element which can be bonded with a larger amount of hydrogen and at least one element which imparts reversibility to the bonding of hydrogen with the element, namely at least one element which makes it possible to desorb hydrogen are combined, in order to reversibly absorb and desorb hydrogen. As the elements which are easily bonded with hydrogen, Zr and Ti are selected, and as the elements which make it possible to desorb hydrogen, Ni and Mn are selected, whereby an alloy forming a Laves structure intermetallic compound is obtained (see, for example, Japanese Patent Kokai Publication Nos. 241652/1985 and 26732/1992).
Ni acts as a catalyst on the alloy surface and improves a rate of electrochemical absorption and desorption of hydrogen, namely the discharge property. Zr contributes to the absorption of hydrogen and has great influence on basic properties of the hydrogen storage alloy such as an amount of absorbed hydrogen, an equilibrium pressure of hydrogen, and the like.
However, the conventional hydrogen storage alloys do not necessarily have satisfactory properties, tend to ignite in the air and have unsatisfactory shapability in the production of electrode.